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How Bulbs Plant Themselves

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Contractile roots pull bulbs down to the proper depth.

For a long time, scientists wondered how bulbs, which grow from seeds that germinate at or near the surface of the soil, end up being buried so deeply. And they discovered that it’s because they produce unique roots called contractile roots. These begin the season like any other root, penetrating deep into the soil, but then they begin to contract, becoming condensed and wrinkled, like an accordion that deflates. Since the lower end of the root clings to the soil particles in its vicinity, the result is that they pull the bulb downward. Thus, the bulb actually plants itself!

The Right Depth

Traditional planting depths for spring-flowering bulbs.

Each species of bulb at its preferred soil depth, one that shelters it from predators and drought, and may take a few years to reach it, descending deeper and deeper each season as the bulb, tiny when it germinates, grows in size. The bulbs even adjust to the type of soil, descending deeper in light, airy soils than in dense, heavy ones.

Studies have shown that the main influence in bulb depth is actually sunlight. Contractile roots react to light penetrating the soil, specifically rays at the blue end of the spectrum, pulling the bulb deep enough into the soil that the penetration of blue rays no longer influences it. And the degree of tolerance to blue rays varies according to the species, explaining why some bulbs descend more deeply than others.

In Your Own Garden

If you pay attention, you can actually observe the phenomenon of contractile roots in your own garden. Here are two examples:

While planting bulbs in the fall, you accidentally drop one and it lies on the ground all winter. (And who hasn’t done that!) You’ll find the lost bulb in the spring, as it will bloom even if it’s only partly buried. But come summer, it will have disappeared entirely, pulled underground by its contractile roots. Within two or three years, it will have planted itself at the right depth for the type of bulb in question.

Or you planted lily bulbs at a depth of 6 inches (15 cm), as recommended on the planting instructions that came with them. Now, some lilies adapt very well to that depth and will remain there. However, when you decide to divide your lilies five years later, you will find that some have “migrated” to a depth of 8 inches (20 cm) and others to a full foot (30 cm), according to their true preferred depth.

Tulips Like It Deep

Most tulips actually like much deeper plantings than bulb companies usually recommend: some botanical tulips will descend to a depth of 2 feet (60 cm) over the years!

Bulbs often pull themselves down below depths where predators can reach them.

The theory for this extreme depth is that the bulb is trying to protect itself from bulb-loving marmots found in their natural environment. The squirrels of your own garden also like tulip bulbs, but they won’t dig anywhere near 2 feet (60 cm) to find them. They’ll even give up past 8 inches (20 cm)! That’s one of the reasons why tulip bulbs planted 1 foot (30 cm) deep often perennialize better than the tulip bulbs planted at the usually recommended 6 inches (15 cm). (Read more about this phenomenon at Deep Planting Prolongs Tulips.)